The present invention relates in general to the field of integrated circuits, and more particularly to bipolar and heterojunction bipolar transistor structures and a method of fabricating such structures.
Bipolar transistors are important components in, for example, logic circuits, communications systems, and microwave devices. A bipolar transistor is essentially a three terminal device having three regions, an emitter, base and collector region, wherein the emitter and collector regions are of one conductivity type and the base is of another.
Since the advent of bipolar transistors, many attempts have been made to improve the performance of the transistor. Some of these attempts have focused on increasing the current gain by improving the injection efficiency of the minority carriers from the emitter to the base. In order to accomplish this, wide bandgap transistors have been fabricated wherein the bandgap of the emitter is wider than the bandgap of the base. A wide bandgap transistor has been constructed in a variety of fashions, and one such transistor structure is referred to as a heterojunction bipolar transistor.
For example, in one type of heterojunction transistor, a transistor is formed with a silicon collector region, a base region composed of a silicon-germanium (SiGe) alloy, and a silicon emitter region. The mixed crystal semiconductor base layer may have a uniform distribution of germanium in silicon or may contain a graded distribution of germanium in the silicon. The graded SiGe distribution is provided in order to increase the bandgap of the transistor. With the above described structure of a wide bandgap transistor and the fact that germanium has a large electron mobility, performance of the transistor is enhanced.
A brief explanation of how the SiGe base region enhances transistor performance is provided briefly below. Graded junctions are used in solid state transistors to enhance device performance. The application of a graded junction in the base region of a bipolar transistor, for example, results in the formation of a nonuniform energy gap. A graded bandgap can be employed to reduce base transit time and thus increase the device speed. More particularly, the bandgap of silicon can be varied by the introduction of dopants, the formation of alloys (e.g., SiGe), and/or the introduction of strain into the crystal lattice. Combinations of all three of these phenomena have been used to produce very high-speed graded SiGe-base heterojunction bipolar transistors. In addition, such graded profile heterojunction transistors may exhibit additional advantages over conventional silicon devices for high speed digital and microwave devices, for example, by providing higher emitter injection efficiency, lower base resistance, lower base transit times, and superior low temperature speed and gain.
Many attempts in further improving heterojunction bipolar transistor performance have focused on, for example, decreasing the width of the base region of a transistor and decreasing the base transit time for a constant base width. Nevertheless, accomplishing such a base thickness reduction has proved difficult for a variety of technical reasons. For example, anneal processes involved in polysilicon type emitter PNP bipolar devices may result in the P-type emitter dopant (e.g., boron) diffusing into the base region. To mitigate the negative impact of such diffusion, an intrinsic silicon buffer layer is typically added to the graded profile base region to prevent such diffusion from negatively impacting the SiGe alloy. Such a buffer layer, however, causes the effective emitter/base epitaxy thickness to increase. Therefore, in conventional heterojunction structures and methods, there is a limitation as to how thin the emitter/base epitaxy can be fabricated when the emitter is formed and contacted by polysilicon.
There is a need in the art for improved structures and methods relating to heterojunction bipolar transistors.
The present invention relates generally to a bipolar transistor structure having a more abrupt emitter-base junction which reduces the base transit time and hence higher transition frequency and gain.
The present invention is directed to a heterojunction bipolar transistor structure having a diffusion blocking layer (e.g., a SiC alloy) associated with the emitter/base region of the transistor. The diffusion blocking layer is operable to retard a diffusion of dopants therethrough from the emitter region down to a SiGe graded profile base layer. The reduction in diffusion allows a buffer layer associated with the emitter/base epi region to be reduced substantially, thereby reducing the emitter/base epi thickness. The emitter/base epi thickness reduction advantageously improves transistor performance and reduces the lattice strain in the base, thereby permitting a peak germanium concentration in the SiGe graded profile base layer to be increased, which advantageously increases the transistor speed and/or gain.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.